The present invention relates to a developing method for visualizing an electrostatic latent image formed on an image-bearing member surface with a magnetic developer including a magnetic toner and magnetic carrier in an electrophotographic method, an electrostatic recording method, and an electrostatic printing method, etc.
Conventionally, in a typical electrophotographic method, an electrostatic latent image formed on a photosensitive material surface is visualized with colored resin particles called toners, and the resulting toner image is fixed to a transfer sheet such as a plain paper by heating or pressurizing means to obtain a fixed image. Various methods of developing such an electrostatic latent image have been proposed so far. Among them, there is a method in which magnetic toner consisting of toner particles comprising a binder resin and magnetic powder is supplied onto a non-magnetic sleeve, to form a magnetic brush on the sleeve by a relative rotation of the sleeve and a permanent magnet member disposed inside the sleeve, and an image-bearing member surface is in slidable contact with this magnetic brush, thereby permitting the toner particles to attach to the electrostatic latent image.
In this one-component magnetic toner system, a chargeable magnetic toner, which contains a charge-controlling agent so that the toner may be strongly charged in a particular (positive or negative) polarity, is used, and the development of an electrostatic latent image is carried out by utilizing a triboelectric charging phenomenon due to the contact of the toner particles with a sleeve or a doctor blade member. However, when the chargeable magnetic toner is used alone, the toner particles are likely to be agglomerated by electric charging, so that streaks are formed on the image due to a shortage of toner on the sleeve. To obviate this problem, it has been proposed to use a developer comprising magnetic toner and magnetic carrier (for instance, U.S. Pat. Nos. 4,640,880 and 5,053,305).
In such a method of developing an electrostatic latent image by using a developer comprising a chargeable magnetic toner and a magnetic carrier, it is typical that a magnetic carrier having a relatively high conductivity is used, and that the toner concentration is not particularly controlled. Accordingly, the toner concentration is in a wide range of 10-90% by weight in a developing region which is formed between the image-bearing member and the sleeve. When a permanent magnet material is fixed and a sleeve is rotated around it, an image quality is greatly affected by the toner concentration. Therefore, in this case, the toner concentration should be restricted to 30 weight % or less by utilizing toner-controlling means.
Also, since the magnetic toner has an insulating property from the viewpoint of transferability, the electric resistance of the entire developer becomes extremely high as the toner concentration increases. Thus, in order to obtain an image having a high image density and a good quality by using such a developer, efficient development of an electrostatic latent image is required. For this purpose, at least one of the non-magnetic sleeve and the permanent magnet member is usually rotated at a high speed to increase a speed of conveying the developer, thereby improving the development efficiency.
However, when either one of the above members is rotated at a high speed, the developer is likely to be severely damaged which can shorten its life, and large noises tend to be generated in the driving system. There also is a problem that in a case where foreign particles such as paper powder, etc., are accumulated at a doctor blade member, streaks are formed on the image due to a shortage of toner on the sleeve, thereby decreasing the image quality.